In one aspect, this invention relates to a seismic device. In another aspect, this invention relates to generating seismic signals in a liquid medium.
In prospecting in subsea and other areas underlying a body of water, it is desirable to provide a source of energy for propagating sonic pulses or shock waves into the water. Since water is a good conductor of sound, it is normally not necessary to generate pulses near the floor of the water body; the pulses can be, and desirably are, produced near the water surface. These pulses propagate down through the water, across the water-floor interface, into the subfloor geologic formations and are, to some extent, reflected back along the same path to an array of hydrophones positioned near the surface of the water. Analysis of the signals produced by the hydrophones can provide information concerning the structure of the subfloor geological formations and the potential for petroleum accumulations in these formations.
An air gun using high-pressure compressed air can be used to generate the seismic pulses. The gun is fired by explosively releasing compressed air from an exhaust chamber into the water.
The frequency of the acoustic output of the gun is an important design consideration and is dependent upon the volume of the exhaust chamber. It is desirable that the output of the gun have a high energy yield in a useful seismic frequency. Exhaust chamber volumes in the range of 10 cubic inches up to about 1,000 cubic inches are commonly used. In many instances, it is desirable to introduce acoustic energy over a wide range of frequency into the subsea formations in order to obtain more information from the reflected pulses. An array of air guns of different exhaust chamber volumes is thus sometimes used. A technique for easily altering exhaust chamber volume to best suit the specific environment encountered would be very desirable.
It has been further found that some air guns accumulate sea water during use if oriented in the vertical or near vertical position. As the air gun chamber gradually fills with sea water, the acoustic power of the gun decreases and the output frequency of the gun increases, changing the tuning of the gun. Since a plurality of guns are normally employed to generate signals, the change in tune of each gun filling with water complicates deconvolution of the return seismic signals. An air gun designed to prevent sea water accumulation would thus be very desirable.